Cutting tree, first of all, bring in chainsaw (7)
I believe the answer is:
acerbic
'cutting' is the definition.
(I've seen this before)
'tree first of all bring in chainsaw' is the wordplay.
'tree' becomes 'acer' (acer is a type of tree).
'first of all' suggests taking the first letters.
The first letters of 'bring in chainsaw' is 'bic'.
'acer'+'bic'='ACERBIC'
(Other definitions for acerbic that I've seen before include "Sour; sharp" , "Bitter (in speech)" , "Astringent in tone" , "Tasting bitter; forthright" , "Sour in taste or harsh in tone" .)